It is with this idea in mind that Suzuki teachers tend to size a student down rather than up on instruments. The teacher's goal is to give the student the tools to play fluently or, as Shinichi Suzuki used to always say, "with beautiful heart." This does not happen if the student is struggling to support the instrument itself.
Fractional instrument size should imitate how the instrument is held by an adult with a full size instrument. This means that wrists should not have to be hyperextended or torsos twisted into painful shapes in order to produce a sound.